1. Field of the Invention
The present invention relates to an optical head that focuses a beam irradiated from the light source in an information recording surface of an optical disc through a transparent base plate on the recording surface to record and reproduce an information signal on and from the recording surface.
2. Description of the Prior Art
Generally, in an optical disc system optical head that focuses a beam irradiated from the light source in an information recording surface of an optical disc through a transparent base plate on the recording surface, an information signal is recorded on or reproduced from the recording surface. The spot size of the beam on the recording surface is preferably small enough to obtain a good property for recording and reproducing.
The more homogeneous is the strength of the beam being incident on an objective lens, the smaller the spot size is tightened. However, the light irradiated from a semiconductor laser used as a laser source for the optical head generally has a Gaussian distribution intensity. Therefore, the truncation of the objective lens is increased to render the optical intensity at effective radius of the objective lens to be close to the center intensity, so that the beam intensity distribution can become homogeneous, as shown in FIG. 1. In FIG. 1, the horizontal line represents rim intensity, i.e., the ratio of the optical intensity at effective radius of the objective lens to the center optical intensity, and the vertical line represents the spot size, which is 1 when the rim intensity is 0 (zero). Referring to FIG. 1, the higher the rim intensity is, that is, the larger the truncation of the objective lens becomes, the smaller the spot size is tightened. The rim intensity depends on the size of the incident beam against the effective radius of the objective lens. In this optical system wherein the beam from the light source is collimated into parallel rays by a collimator lens and the parallel rays are subsequently incident on the objective lens, the size of incident beam is proportional to a focal length of the collimator lens. Therefore, the focal length will be determined so as to obtain the desired rim intensity with the objective lens.
The cross section of the spot can not become a true circle, where the rim intensity changes in a circumferential direction, since the rim intensity and the spot size have such a relationship as shown in FIG. 1. The Intensity distribution of the irradiation ray in a horizontal direction parallel to the junction face of the semiconductor laser differs from the intensity distribution of the irradiation ray in a vertical direction orthogonal thereto, so far as the Gaussian distribution of the irradiation ray is concerned. If the angle of full width at half maximum (hereinafter called as F.W.H.M.) in the horizontal direction and that in the vertical direction is expressed by θh1, and θv1 respectively, the ratio θh/θv is generally within the range from ½ to ⅓ and, therefore, the cross sectional shape of the beam becomes a long ellipse in the vertical direction. When the elliptic beam is converged by the objective lens, the rim intensity in the horizontal direction becomes lower than that in the vertical direction, and the shape of a beam spot on the surface of the optical disc becomes an ellipse, which has a spot size in the horizontal direction that is larger than in the vertical direction. If it is necessary to rectify the elliptic beam spot to become a beamspot of a true circle, an optical beam shaping system that closes the beam size in the horizontal direction with the beam size in the vertical direction is used. For example, two prisms may be used as the optical beam shaping system as shown in FIG. 2. Referring to FIG. 2, a beam 12 is irradiated from a semiconductor laser 11, and is then collimated to parallel rays by a collimator lens 13 before the collimated beam 12 is successively transmitted through a prism 14 and a prism 15. The prism 14 magnifies the beam size from D1 to D2 on a plane parallel to the surface of the paper, but does not magnify the beam on the plane perpendicular to the surface of the paper. Note that the prism 15 magnifies the beam size from D2 to D3 as shown in FIG. 2, however, the magnification ratio D3/D2 of the prism 15 is as well as the ratio D2/D1 of the prism 14. Therefore, adjusting the horizontal direction of the junction plane of the semiconductor laser 11 to become parallel to the surface of the paper results in beam shaping. The magnification may be determined in order to obtain a desired spot shape.
As above mentioned, it is preferable to shape and extend the focal length of the collimator lens for the optical head. However, the collimator lens acts to increase the amount of the beam fluxes vignetted to thereby decrease the efficiency of utilization of the beam. Considering the rim intensity and the efficiency of utilization of the beam, a feasibly balanced combination of the focal length of the collimator lens 13 and the magnification of the prism should be selected. This effect will be explained with reference to FIG. 3. In FIG. 3, the horizontal line represents the magnification of beam shaping and the vertical line represents the focal length fCL of the collimator lens. In the example shown in FIG. 3, the angle of F.W.H.M. in the horizontal direction and that in the vertical direction are θh=11° and θv=27°, respectively. The effective radius of the objective lens is 3.4 mm. Referring to FIG. 3, the curve A is in the case of the 35% rim intensity in the horizontal direction, the curve B is in the case of the 40% rim intensity in the vertical direction, and the curve C is in the case of the efficiency η=45% of utilization of the beam.
According to the above principle, the above curve A and curve B, and below curve C, is indicated by hatching in FIG. 3. If a combination of the focal length fCL and the magnification M of beam shaping is selected from the area, then the combination can satisfy the condition of the 35% rim intensity in the horizontal direction, the 40% rim intensity in the vertical direction, and the 45% efficiency of utilization of the beam. When the magnification M of beam shaping is lower than 2.5, the cross sectional shape of the collimated rays cannot become a true circle. Referring to FIG. 1, however, because the spot size will change a small amount at a rim intensity larger than 20% or 30%, the optical head having a good performance can be provided.
By the way, in the optical beam shaping system as shown in FIG. 2, an astigmatism will be caused, when the beam being incident on prism 14 and 15 is not parallel rays. When the semiconductor laser 11 is displaced from the focal point of the collimator lens 13, the beam may not be parallel rays, so that the astigmatism will be caused. FIG. 4 shows a simulation of relationship between a displacement of the laser source and the astigmatism. The semiconductor laser source irradiates a laser having a wavelength 650 nm, and an angle of F.W.H.M. θh=11° in the horizontal direction and θv=27° in the vertical direction, respectively. The magnification M of beam shaping is 2.5, and the focal length fCL=8.0 mm. Referring to FIG. 4, the astigmatism is proportional to the displacement of the laser source, and the proportional coefficient is 5.7 mλ/μm. If the astigmatism is 30 mλ, in order to have no influence against recording and reproducing the signal, the maximum permissible displacement will be about 5.3 μm. An interval between the laser source and the collimator lens may change with temperature. Assuming that a base stand supporting the optical member is made of an aluminum alloy, and a temperature range ΔT in which the performance of an optical head is warranted is ΔT=±30 degrees, the displacement ΔZ of the interval can be estimated as follows.ΔZ=fCL·ΔT·α=5.3 μm
Note that α is an expansion coefficient of the aluminum alloy which is 2.3×10−5 degree−1. The displacement ΔZ may be equal to the maximum permissible displacement, and therefore we consider the astigmatism caused by temperature shift may be permitted.
In this case of the wavelength of about 650 nm, the good spot size and the good efficiency of utilization of the beam can be compatible and the astigmatism of the optical beam shaping system can be in the permissible range.
Recently, the short wavelength laser source having a wavelength not longer than 500 nm is developed to realize the optical disc having a high density recording. When the short wavelength laser source is used in the optical head system, the wave front aberration is inversely proportional to the wavelength, then the astigmatism will be made larger than at a wavelength of about 650 nm. For example, when a semiconductor violet laser having a wavelength 400 nm is used, the astigmatism will be about 1.6 times as large as at wavelength of 650 nm. Therefore, the maximum permissible displacement ΔZ of interval between the source and the collimator lens will be 0.6 times lower than at a wavelength of 650 nm. When beam shaping is performed on the same condition, the astigmatism caused by the displacement of the interval between the source and the collimator lens, and then particularly, the recording and reproducing performance will be remarkably dropped due to the temperature shift.
Japanese Patent No. 2,933,325 shows the beam shaping system, which adjusts a location of the collimator lens by positively making use of the displacement of the interval between the source and the collimator lens, so that the astigmatism can be compensated for. However, this system can be applied to only stable astigmatism, the variable astigmatism due to temperature shift can not be cancelled by this system. If a drive system of the collimator lens in the optical axis will be provided, then temperature shift may be compensated, but this will result not only in increase of the number of members, but also the necessity of both a servo circuit for controlling the location of the lens and a studying program for adjusting the lens.